To assess one's physical capability, it is usually not sufficient to track one's time or speed alone. Wind resistance, elevation, road conditions, or weight can vary from one bicycle ride to the next. It is highly useful to be able to track power as a metric for a bicycle rider's performance. Prior art has described a means of measuring power through the use of sensors attached to a bicycle front sprocket [U.S. Pat. No. 7,047,817], attached to a bicycle hub [U.S. Pat. No. 6,418,797], attached to a chain [U.S. Pat. No. 5,167,159], and attached near a chain to measure vibrational frequency of the bicycle chain [U.S. Pat. No. 6,356,848]. Customized sprockets or hubs require significant modification to the bicycle and are currently quite costly and not suitable for economically minded bicycle riders or youths.
Devices intended to measure chain tension using additional wheels or sprockets [U.S. Pat. No. 5,167,159] have been too large and bulky to achieve widespread use. Many bicycles have <1.5 cm of clearance below the lower surface of the upper path of the bicycle chain. This does not allow sufficient space for sprockets and would severely restrict ability to access gears on most speed bicycles. Additionally, wheel- and sprocket-based devices using radii <=2.5 cm have suffered from high energy dissipation and noisy operation due to the polygon effect (an effect caused by the varying chain path-length as the chain moves around the sprocket). The invention presented in U.S. Pat. No. 5,167,159 is unsuitable because of these two problems.
The system based on measuring chain vibration [U.S. Pat. No. 6,356,848] has involved complex signal processing and has been deemed not useful when riding on surfaces that are bumpy (non-tarmac) and therefore has not been widely adopted. The system based on U.S. Pat. No. 6,356,848 has poor effectiveness because it is based on a structure that is not in direct contact with the chain but is instead some distance away from the chain. Additionally, prior disclosed bicycle power measurement systems have used visual displays to report the power to the rider. It is undesirable in many cases for a rider to take their eyes off the surrounding environment in order to monitor the power reading. It is useful for the rider to be able to get this power information through an auditory report or even tactile actuator. For these reasons, it is desirable to have a bicycle power measurement system that is cheaper and does not require any replacement or customization of existing bicycle components. It is desirable to have a system that can be implemented by attaching an additional, light-weight apparatus onto the existing bicycle and can directly measure the power expended and report information without much distraction to the rider.
For prior configurations based on strain gauge measurement, the rider is usually applying less than 300 Newton (Nt.) of force (less than 70 lbs.) to the chain for most of the riding time period. However, it is possible for riders of weight 90 kg (200 lbs.) to attain additional force and put up to 1300-1800 Nt. (300-400 lbs.) on the pedal assembly. With an additional factor of the 3-to-1 mechanical advantage of the crankshaft, a 90 kg rider can put up to 4000-5400 Nt. (900-1200 lbs.) of tension on to the chain for brief periods. This potentially large dynamic range leads to low sensitivity and accuracy for the majority of the bicycle riding period. The majority of time is spent with 300 Nt. (70 lbs.) or less. When device strain is directly proportional to chain tension, percentage error is inferior for the majority of time (for less than 300 Nt). It is beneficial if one can achieve comparable percentage error over the low- and high-force periods of the ride.